1. Field of the Invention
This invention relates generally to circuits which are sensitive to waveform transition and which are capable of delivering an output to a utilization circuit during only one of two waveform transitions of an applied input pulse. More specifically, it relates to waveform transition sensitive circuits which utilize Josephson junctions and the ability of the latter to switch regardless of current direction to apply either an additive or substractive current to a current already flowing in another Josephson device, the switching of which can deliver a current to a utilization device.
Still more specifically, it relates to the incorporation of such waveform transition sensitive circuits in a sense bus for Josephson random access memory arrays. Sensing is carried out in the absence of timing signals and, because the circuit can be powered by pulsed current, resetting of the sense bus is automatic. Still more specifically, it relates to logic circuits such as AND, OR circuits which may be developed by using a plurality of input circuits all of which provide currents which may be added to or subtracted from a current flowing in a Josephson device which can be delivered to a utilization circuit and represent a desired logic function depending upon how the various currents are manipulated. The resulting circuits which are preferably latching in character provide advantages which include reasonably wide margins, fast operation and, in the instance of the sense bus, a reduction in access time.
2. Description of the Prior Art
U.S. Pat. No. 3,292,160 in the name of Crittenden filed June 6, 1957 shows a circuit which includes a pair of cryotrons disposed in parallel. Using appropriate circuitry, circulating currents of opposite polarity can be set up in the loop. Their polarity can be sensed by applying a negative sense pulse to the loop to which the circulating currents add or subtract. Where the applied current is additive with the circulating current, it causes one of the cryotrons to switch and provides a voltage pulse to a sense circuit. Where the applied pulse is subtractive, no switching occurs and no output pulse is provided. This circuit, while it shows the addition and subtraction of a circulating current with another current, it is not responsive to the waveform transitions of a single pulse which, with a transition of one direction, produces no output but, with a transition of the other direction, does produce an output. In the patent, circulating current is already present, whereas in the present application, no circulating currents need be involved and the present circuit is responsive to waveform transitions whereas the circuit of the patent is responsive to the overall polarities of the applied pulses which set up either clockwise or counterclockwise circulating currents in a superconducting loop.
U.S. Pat. No. 2,949,602 in the name of Crowe filed Apr. 11, 1958 utilizes two cryotrons in a single loop. The application of a rising input induces circulating currents in the loop, switching one of the cryotrons and producing an output pulse of one polarity. If the input is applied having opposite polarity, opposite polarity output pulses are produced. The circuit of Crowe is transition sensitive, but operates in the same manner for rising and falling currents. While its outputs are repre;sentative of the transition direction of the inputs, it does not have the capability of switching for one transition of input current but not for the other transition as does the present circuit.
U.S. Pat. No. 3,987,309 in the name of Hamel et al filed Dec. 23, 1974 shows a sensing circuit which works on the principal of shunting gate currents from one branch into another by causing the Josephson junction to switch. At a later time, logic inputs propagate current toward the Josephson device which is now in the zero voltage state having reset after switching. The current in the Josephson device which is now due to the switching of other logic devices acts as a control current for a Josephson sense device which switches when gate current is applied to the Josephson sense device. This sense circuit is not responsive to the waveform transitions of an input signal.
U.S. Pat. No. 3,164,808 in the name of Rogers et al filed May 2, 1960 shows a switchable device shunted by a loop containing an inductance. The arrangement works on the basis of having circulating current or no circulating current in the loop to represent binary states. To read information from a cell, a pulse is applied which switches the switchable device (cryotron) interrupting circulating current and causing it to decay with a time constant L/R. If there is no circulating current in the loop, there is no output. This circuit is not responsive to the waveform transitions of a single input pulse. Rogers et al like Crowe induces circulating currents; the direction of which is a function of the rise or fall of a signal applied to a superconducting loop. Circulating currents can only appear when the switchable device is not resistive.
IBM Technical Disclosure Bulletin, Vol. 17, No. 1, June 1974, p. 259 in an article by W. Anacker et al entitled, "Transformer Coupled Power Supply for Josephson Tunneling Logic Circuits" shows the transformer coupling of power into logic circuits. A loop contains a pair of Josephson devices both of which are subject to the same gate currents by transformer coupling. One of the devices has a diode-like characteristic which is utilized to eliminate circulating currents so that the logic device can be reset. The diode device interrupts the superconductivity of a loop for current flowing counterclockwise while it remains superconducting for current flowing clockwise. This circuit is not responsive to the waveform transitions of an applied input as can be seen from the fact that the loop device of the present application switches for both waveform transitions.